Only revert commits in the same repository

Summary: Fixes T8215. When adding a revert edge, only consider commits within the same repository.

Test Plan: Imported the arcanist repository three times and inspected the reverts relationships on rARC490984936bb7ddfc589ef3d376ec2ac2365f070d.

Reviewers: hach-que, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8215

Differential Revision: https://secure.phabricator.com/D12875